In an Unmanned Ground System (UGS) network, the Command and Control (C2) vehicle initiates formation of a mobile ad hoc wireless sensor network. One type of network architecture for a UGS network is a tree-based architecture. The tree-based network architecture takes advantage of the fact that sensor nodes are relatively static and, therefore, do not require frequent routing updates. The C2 vehicle (also called the C2 node), on the other hand, can leave and form the network at any time and any location.
The C2 node generates two important commands: an “Activate” command or message to form the network and a “Deactivate” command or message to dissolve the network. The C2 node initiates formation of the UGS network by broadcasting the “Activate” command, and can re-route the existing with the “Activate” command. When the C2 node leaves the UGS network, it broadcasts the “Deactivate” command to sensor nodes so that they reset their stored network topology data in preparation to receive the next “Activate” command.
Broadcasting is a common operation in mobile ad hoc networks (MANETs). Broadcasting is used in routing protocols to discover routes and in group communication operations. Broadcasting schemes for MANETs fall into four general categories: Blind flooding, area-based methods, probabilistic scheme, and neighborhood-knowledge-based methods. In flooding, a broadcast message is forwarded exactly once by every node in the network. Flooding is wasteful, especially in dense networks. Several efforts have targeted reducing the redundancy of flooding broadcasts.
In a blind flooding method each node receiving a copy of a broadcast message first checks whether it has already received it. If so, the node silently discards the message. If the message was received by the node for the first time then the node rebroadcasts it to all of its neighbors. Schemes such as probabilistic, counter-based, distance-based, and cluster-based, have been proposed to alleviate the drawbacks of redundant rebroadcasts.
In a probabilistic re-broadcasting approach, upon receiving a broadcast message for the first time, a node will rebroadcast it with probability P similar to the gossiping-based approach. The adaptive gossip probability is explored to make the protocol insensitive to a changing environment. However, the reliability of this approach depends on the probability P and the topology of the network.
A number of neighborhood knowledge-based algorithms use the connectivity of the node's k-hop neighbor information for some small integer (typically 2). The algorithms use this partial topology knowledge to reduce the number of nodes which relay the broadcast message. Two related solutions have been proposed: Self-Pruning and Dominant-Pruning. In the self-pruning-based broadcast protocol, each node collects neighborhood topology information (i.e., static information) by exchanging “Hello” message and extracts broadcast history information (i.e., dynamic information) from incoming broadcast messages. Each node decides its role in a specific broadcasting event: it is a forward node and forwards the broadcast message, or a non-forward node (i.e., is self-pruned) and discards the message silently. Collectively, forward nodes, including the source node, form a connected dominating set and ensure the coverage. A set of nodes is a dominating set if every node in the network is either in the set or a neighbor of a node in the set. Nodes are selected into this set in such a way that they cover a node's entire two-hop neighborhood. This requires substantial communication between neighboring nodes to update the neighbor information.
There are several drawbacks associated with using existing broadcast protocols in a UGS network. First, the existing broadcast protocols focus only on the efficient delivery of broadcast message, that is maximizing the probability the message reaches all nodes in the network. Impact of the significant network topology changes that are initiated by the received broadcast message is not considered in the conventional broadcast protocols. Second, the conventional broadcast protocols require large amount of information about the topological or geographical structure of the network when the network topology is changing. This creates significant overhead problems in a sensor network. Third, conventional broadcast and routing techniques are based on optimization at the individual layer (namely link layer or network layer). As a result, neighbor discovery creates racing problems in the network routing updates. Finally, conventional broadcast techniques lead to unwanted power consumption in the nodes which is a significant problem in battery-operated nodes.
There is room for significant improvement in broadcast protocols for use in wireless networks, particularly wireless sensor networks.